These essential amino acids - branched chain amino acids or BCAAs - are not produced naturally in the human body and must be acquired through food.

Because of the speeded up metabolism, the patients with this mutation burned through their stores of the amino acid more quickly than normal, creating an imbalance the researchers think may be linked to the neuro-behaviorial symptoms of autism.

"It was very surprising to find mutations in a potentially treatable metabolic pathway specific for autism," said co-author Joseph Gleeson of the University of California, San Diego.

"What was most exciting was that the potential treatment is obvious and simple: Just give affected patients the naturally occurring amino acids their bodies lack."

The genetic mutations were identified after researchers studied the genomes of two closely related families with children who have autism and epileptic seizures.

Mice study

Gleeson's team then genetically engineered mice to test the impact of the dietary supplement.

The mice with the genetic mutation displayed autism-like symptoms, including spontaneous epileptic seizures, but their condition improved when treated with the supplements.

"Studying the animals was key to our discovery," says first author Gaia Novarino who works with Gleeson.

"Once we found that we could treat the condition in mice, the pressing question was whether we could effectively treat our patients."

The researchers used the supplement to correct the amino acid imbalance in the human patients, but they say there is not enough data to determine whether the treatment improved their autism symptoms.

Roughly a quarter of autism patients have epileptic seizures, but Gleeson said this particular condition is believed to be "extremely rare."

While researchers do not yet understand why this works, the identification of the genetic mutation can also help establish a basis for screening autism and epilepsy patients.